I'm modeling a simple hard surface object, not using SDS. But I need a couple of edges to be well rounded, so I beveled them with 3 segments and smooth-shaded the bevel faces. It rounds well enough, except that it doesn't blend with the other faces, there is a hard seam between the smooth-shaded and flat-shaded faces. Is there a way to fix this? To blend the two together? I need the rest of the faces flat-shaded. So I need to combine the two.
In many cases it's better not to use different shading types, rather than set whole mesh as smooth.
For the rest of the mesh which is intended to be flat you either add Edge Split modifier or check Auto Smooth in Data tab > Normals and set the andgle you need. Then parts of the mesh will be smooth, others - flat:
When joining several meshes with different split angles used
In this case it may be easier to use Edge Split modifier and apply it. It will create double vertices that will make shading look depending on angle you've selected and after joining meshes even with different angle of splitting normals they all will continue look like they did before.
In other cases
You may want to use Auto Smooth as there will be no extra modifier in stack and no double vertices so the mesh will be easy to edit.